Partially-depleted silicon-on-insulator (PDSOI) MOSFETs offer high speed and low power performance, but typically remain susceptible to parasitic floating body effects (FBE) which can seriously degrade device performance. Various techniques have been proposed for reducing FBE in SO MOSFETs. One such technique includes using a narrow bandgap SiGe layer adjacent a source of an SOI NMOS field effect transistor. As will be understood by those skilled in the art, the use of a SiGe layer reduces the potential barrier for holes passing from the body region to the source region. Therefore, holes generated in the body region by impact ionization can more readily flow into the source region through the path of the p-Si(body)/n+SiGe(source)/n+Si(source). This and other related techniques are disclosed in articles by J. Sim et al. entitled “Elimination of Parasitic Bipolar-Induced Breakdown Effects in Ultra-Thin SOI MOSFETs Using Narrow-Bandgap-Source (NBS) Structure,” IEEE Trans. Elec. Dev., Vol. 42, No. 8, pp. 1495-1502, August (1995) and M. Yoshimi et al. entitled “Suppression of the Floating-Body Effect in SOI MOSFETs by the Bandgap Engineering Method Using a Si1-xGex Source Structure,” IEEE Trans. Elec. Dev., Vol. 44, No. 3, pp. 423-429, March (1997). U.S. Pat. No. 5,698,869 to Yoshimi et al. entitled “Insulated-Gate Transistor Having Narrow-Bandgap-Source” also discloses the use of a narrow bandgap material within a source region of a MOSFET.
Techniques to reduce FBE and improve channel characteristics in MOSFETs are also described in U.S. Pat. No. 5,891,769 to Liaw et al. entitled “Method for Forming a Semiconductor Device Having a Heteroepitaxial Layer.” In particular, the '769 patent discloses the use of a strained channel region to enhance carrier mobility within MOSFETs. This strained channel region may be formed by growing a silicon layer on an as-grown relaxed or unstrained SiGe layer. U.S. Pat. No. 5,963,817 to Chu et al. entitled “Bulk and Strained Silicon on Insulator Using Selective Oxidation,” also discloses the use of SiGe layers, which selectively oxidize at faster rates relative to silicon, to improve FBE. Furthermore, U.S. Pat. Nos. 5,906,951 and 6,059,895 to Chu et al. disclose wafer bonding techniques and strained SiGe layers to provide SOI substrates. The use of wafer bonding techniques and SiGe layers to provide SOI substrates are also described in U.S. Pat. Nos. 5,218,213 and 5,240,876 to Gaul et al. Conventional techniques for forming SOI substrates are also illustrated by FIGS. 1A-1D and 2A-2D. In particular, FIG. 1A illustrates the formation of a handling substrate having a porous silicon layer therein and an epitaxial silicon layer thereon and FIG. 1B illustrates the bonding of a supporting substrate to a surface of the epitaxial silicon layer. The supporting substrate may include an oxide layer thereon which is bonded directly to the epitaxial silicon layer using conventional techniques. As illustrated by FIG. 1C, a portion of the handling substrate is then removed to expose the porous silicon layer. This removal step may be performed by grinding or etching away a portion of the handling substrate or splitting the porous silicon layer. As illustrated by FIG. 1D, a conventional planarization technique may then be performed to remove the porous silicon layer and provide an SOI substrate having a polished silicon layer thereon and a buried oxide layer therein. The conventional technique illustrated by FIGS. 1A-1D is commonly referred to as an epi-layer transfer (ELTRAN) technique. FIG. 2A illustrates a step of forming a handling substrate having a silicon layer thereon by implanting hydrogen ions into a surface of the substrate to define a buried hydrogen implant layer therein. Then, as illustrated by FIG. 2B, a supporting substrate is bonded to the handling substrate. A portion of the handling substrate is then removed by splitting the bonded substrate along the hydrogen implant layer, as illustrated by FIG. 2C. A conventional planarization technique may then be performed to remove the hydrogen implant layer, as illustrated by FIG. 2D. The conventional technique illustrated by FIGS. 2A-2D is commonly referred to as a “smart-cut” technique.
Unfortunately, although the use of strained silicon channel regions may enhance carrier mobility in both NMOS and PMOS devices, such strained regions typically degrade short channel device characteristics. Thus, notwithstanding the above-described techniques for forming MOSFETs and SOI substrates, there continues to be a need for improved methods of forming these structures that do not require the use of strained channel regions to obtain enhanced channel mobility characteristics, and structures formed thereby.